1. Field of Use
This invention relates to data communication systems, and more particularly, to data communication modems.
2. Prior Art--General
With the proliferation of digital computers in stand-alone and in interactive environments, a rising need has developed for digital communications between remote computers. This need has been met, for the most part, by utilizing the existing network of analog voice-bandwidth channels employed for voice communications, i.e., the telephone network. To communicate digital signals over these analog channels which have a pass bandwidth of 300-3000 Hz, it is necessary to appropriately translate the spectrum of the transmitted digital signals into the allowable band and, upon reception, to appropriately recover the digital signals from the spectrum-translated transmitted signals. That is, it is necessary to MODulate the sent digital data with a voice-frequency carrier signal, and to DEModulate the received signal to recover the sent digital data. A digital transceiver, or data set, which is capable of the above operations is known as a MODEM.
The techniques and embodiments of data communication modems form a large body of knowledge. Reference is made herein to R. W. Lucky et al Principles of Data Communication, McGraw-Hill, (1968).
In general, a modem (data set) contains a digital signal port connected to a local data terminal, e.g., a TELETYPE.RTM. transreceiver, an analog signal port connected to the transmission medium, e.g., a telephone network, a modulator section, a demodulator section and a line control section. The modulator section modulates the digital signal with a voice-frequency carrier, the demodulator section converts the received modulated analog signal into a digital signal, and the line control section controls the initiation and termination of the data communication in addition to controlling various data terminal functions during the communications process e.g., half duplex or full duplex operation.
Although all data sets have the basic elements enumerated above, there exists a substantial spread in data set features and characteristics which are commercially available. This spread results from the particular needs and requirements of various applications. Included among the varying data set characteristics are: data rates--varying from 75 to 9600 bits per second; modes of operation--asynchronous for low speed data sets (1200 bits per second and lower) and synchronous for high speed data sets; modulation--frequency shift keying modulation (FSK), differential phase shift key modulation (DPSK), quadrature amplitude modulation (QAM), vestigial side band modulation (VSB), etc.; and transmission medium equalization--fixed equalization or adaptive equalization.
Modem users often require a plurality of data sets on their premises, with each of the data sets having different characteristics. Again, presently such requirements are met by installing an individual data set for each desired application. This represents a substantial investment to the customer and to the data set supplier. Also, it often occurs that a user's requirements change from time to time, such as when the user upgrades his communications capability. Again, presently, such upgrading involves the labor and expense of substituting existing data sets with the desired ones. For greater flexibility, therefore, there appears to be a need for a universal data set which can serve the functions of a plurality of data sets and which can be easily altered to provide the particular characteristics of any desired data set.